Gas analysis



Nov. 8, 1960 c. F. ROBINSON ETAL GAS ANALYSIS Filed May 2, 1957 mw A N55 m w m 5 M 3 mm am 4 MM v:v I B 2. N $5. v mmmz xo zo B5 22 mv vn Q 98 9 656 J 5E: 95 W 33... W 5.158 Q\ W QM w Ez w N m United States Patent GAS ANALYSIS Charles F. Robinson, Pasadena, and Wilson M. Brubaker, Arcadia, Calif., assignors, by me'sne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Filed May 2, 1957, Ser. No. 656,695

2 Claims. (Cl. 250-43.5)

invention provides simple and sensitive methods and apparatus for analyzing gas by ionization with electromagnetic radiation, such as ultraviolet light.

The invention is useful in a wide field of gas analysis, such as the detection and measurement of impurities in air, and industrial process streams. The method and apparatus are also useful in analyzing and monitoring gas streams such as those flowing through anesthetic equipment, a vapor-phase chromatographic column, and, in general, in a wide variety of industrial and experimental processm involving gas mixtures.

V The invention is based on the principle that various gaseous components are ionized by electromagnetic radiation of different energies, thus making it possible to ionize preferentially one component in a gaseous mixture in the presence of another component when the two com ponents have different ionization potentials. irradiating the gas mixture with electromagnetic radiation of an energy sufiicient to ionize one of the components but not the other, an ion. current is produced which is indicative not only of the presence of the ionized constituent but also its relative abundance.

In terms of apparatus, the invention contemplates a chamber in which a sample of gas is disposed. Means are provided for irradiating the gas in the chamber with electromagnetic radiation of suflicient energy to ionize at least a part of the gas. Means are provided for measuring and collecting a substantial amount of ions so produced to provide an analysis of the gas.

In one form, the chamber includes means for flowing the gas sample through the chamber, and at the same time, surrounding the flowing stream of gas sample with a shroud of inert gas to reduce spurious ionization which could be produced by the emission of photoelectrons from the walls of the chamber.

In terms of method, the invention contemplates analyzing a gas mixture of at least two constituents having difte-rent ionization potentials by irradiating the gas mixture with the electromagnetic radiation of an energy which is between the ionization potentials of the two constituents so that at least part of the constituent having the lower ionization potential is ionized. Substantially all of the ions so produced are collected and measured to determine the relative abundance of the ionized component. Preferably, the gas mixture is maintained at a-pressure of at least several mm. Hg to increase ionization and the sensitivity of the measurement.

One important application of the invention is in improving gas analysis in vapor-phase chromatography in which a mixture of components in the vapor or gaseous phase is carried by a carrier gas through a column of an adsorbing material such as silica gel or activated charcoal on which the various components are adsorbed to difierent degrees. In a typical chromatographic analysis, the components migrate through the column at different rates, and are separated and detected or collected as they emerge from the column. However, sometimes two dif- {6176M components have similar chromatographic adsorpice tion characteristics, and emerge together from the column, so that conventional chromatographic analysis fails to distinguish between the two similarly adsorbed compounds. In many cases, the two compounds with similar adsorption characteristics have different ionization potentials so that by irradiating the mixture of the two compounds with electromagnetic radiation of suflicient energy to ionize one but not the other, the mixture of the two components can be analyzed. The prior passing of the sample through the adsorption column often provides the advantage of separating components which would interiere mutually during the ultraviolet analysis of the gas.

These and other aspects of the invention will be more fully understood from the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 is a schematic flow diagram of apparatus for practicing the invention in connection with vapor-phase chromatography;

Fig. 2 is a schematic sectional view of one form of an ionization chamber suitable for practicing the invention; and

Fig. 3 is a view taken on line 3-3 of Fig. 2.

Referring to Fig. 1, which discloses apparatus for analyzing a gas sample containing two constituents of similar chromatographic adsorption characteristics but difierent ionization potentials, a carrier gas such as helium flows from a carrier gas reservoir 9 through a line 10, through a flowmeter 1-1, a flow control valve 12, and one section of a conventional chromatograph analyzer or detector 14. Since the detector is conventional, its detailed construction is not shown but, briefly, it includes four thermal conductivity elements which are constructed so that four filaments are connected electrically to form the four arms of a Wheatstone bridge. The carrier gas flows through the detector in contact with two of the filaments in opposite arms of the bridge, and then flows through a line 16 to a sample chamber 18. A bypass line 20 and sample bypass valves 22 are connected across the sample chamber so that the carrier gas can either be bypassed around the sample chamber or directed through it to pick up and carry a sample through a line 24 into a conventional chromatographic column 26, which contains a suitable adsorbent, such as silica gel or activated charcoal. The sample may contain any of a variety of components, but for the purpose of explaining the invention, it is assumed that it contains a cyclic parafl'ln such as cyclohexane, a cyclic olefin such as benzene, and other constituents such as methane and ethylene.

The cyclohexane and benzene have substantially the same retention or migration time in the column and cannot be distinguished from each other by chromatographic analysis alone. Cyclohexane has an ionization potential of 10.22 electron volts (1210 A.), and benzene has an ionization potential of 9.52 electron volts (1290 A.) so the two compounds can be distinguished by preferential ionization of the benzene. Methane has an ionization potential of 12.8 electron volts (967 A.), and ethylene has an ionization potential of 10.5 electron volts (1170 A.). Since methane has a lower ionization potential than benzene, the methane must be removed before the benzene is ionized, or else the methane would produce interfering ions. Both the methane and the ethylene have retention time in the column which are less than the retention times of the cyclohexane and the benzene, so the methane and the ethylene are separated from the cyclohexane and the benzene. Of course, if benzene were the only component of interest and all the other valves are set to bypass the sample chamber so that only carrier gas is fiowing through the column, an analysis of the sample is made by changing the positions of the bypass valves to close the bypass line and direct the flow of carrier gas through the sample chamber. The sample is adsorbed in the column, and remains adsorbed until it is eluted by the additional flow of carrier gas. The retention time of the various sample components in the column depends on their adsorption characteristics. For the example given above, the cyclohexane'and the benzene are retained longer than the other components and finally leave the column together through line 28 and then flow through the detector where they pass over the other two arms of the Wheatstone bridge in the detector. Due to the difference in thermal conductivity between thecarrier gas and the two hydrocarbons, a bridge unbalance is produced which is measured to determine the amount of cyclohexane and benzene in the sample. However, since these two components leave the column together, the detector does not distinguish between the two.

The gas from the column leaves the detector through a line 30 and flows to a valve 32 which can be set either to exhaust the gas or to direct the gas to an ionization chamber 34. Valve 32 is set to pass the mixture of cyclohexane and benzene through the ionization chamber where it is ionized by irradiating it with ultraviolet light having a wave length between 1165 A. and 1290 A., which energy is sufficient to ionize the benzene but not the cyclohexane. The benzene ions are collected and measured by any suitable means, which provides a measure of the amount of benzene in the sample. The amount of cyclohexane is then obtained by difference.

The ionization chamber may take many forms, and one suitable arrangement is shown in Fig. 2. The chamber includes a cylindrical tube 36 closed at each end. A sample inlet tube 38 opens into the center of the inlet end of the chamber and aninert gas line 40 opens into the same end. An annular diffuser 42 is sealed across the interior of the chamber at the inlet end. The diffuser may be of any suitable porous material such as a perforated porcelain disk or a glass wool pad. The inner end of the sample tube 38 is seal-ed in a central opening 44 of the diffuser. Thus, a stream of sample gas is directed longitudinally along the central axis of the chamher, and an annular shroud of inert gas, coaxially disposed around the central stream of sample gas, also flows through the sample chamber. Both the inert gas and the sample gas flow out at opposite ends of the ionization chamber through an outlet line 45, which may }be connected to the inlet of a pump 46 if it is desired to maintain the interior of the ionization chamber at sub-atmospheric pressure. Ordinarily, the pressure in the ionization chamber is maintained well above the freemolecular flow range, i.e., the pressure is kept sufficiently high so that the mean free path of gas molecules is always small compared to the diameter of the ionization chamber. In addition, the rate of gas flow through the chamber is kept in. the laminar flow range to avoid turbulence and mixing of the inert shroud gas with the sample gas. A pair of vertically spaced, elongated and horizontal rectangular plates 47 are disposed in the central portion of the ionization chamber. The spacing between the plates is at least equal to the diameter of the sample gas stream so that substantially all of the sample gas stream flows between the plates. The opposite ends of a. source of DC. potential 48 are connected by leads 49 to plates 47 to establish an electric field across them. An elongated ultraviolet lamp 56 is longitudinally disposed to the left (as viewed in Fig- 3) of the center line of the ionization chamber at the forward ends of the two plates. A suitable source of power 51 is connected by leads 52 to the lamp. The leads 49 and 52 are suitably sealed through the walls of the ionizationchamber. A vertical collimating plate 53 with a horizont'al slit 54 is disposed between the lamp and plates 47.

The slit is dimensioned to prevent any UV. light from striking the plate 47.

As sample gas flows between the plates, the ultraviolet light is turned on so that it emits UV. radiation of sufficient energy to ionize at least a portion of the gas sample. The ions produced are collected on the plates due to the potential gradient between them, and the resulting current is measured by a suitable meter 55 connected in series with the plates.

The use of a shroud of inert gas is useful in cases where the amount of gas sample to be measured is limited, and where the measurement would be disturbed by effects which would occur at the walls of the container. This kind of difliculty is encountered, for example, in measurements of ionization of gases by X-rays or ultraviolet light where wall disturbances are due to electrons ejected from the walls of the ionization chamber or of the lamp by incident radiation. The wall efiects can be reduced by filling a large chamber with sample gas, and using only the inner portion of the sample gas. How ever, in cases where the amount of sample gas to be measured is limited, the necessity of filling a large volume with the gas to measure the properties of only a small portion of the volume causes a loss in sensitivity. This difficulty is avoided when the gas is flowed in a stream and surrounded by a shroud of inert gas, as shown in Fig. 2. In many situations of practical interest, the use of an inner gas shroud around the sample can increase the sensitivity of measurement by a factor of 10 to 100.

It is obvious that the shroud gas should not exhibit any properties analogous to wall effects itself. Thus, if the effect being measured is ultraviolet ionization, helium makes an excellent shroud gas, because its ionization potential is of the order of 25 electron volts, which is considerably higher than any component of interest. Even if the ionization potentials of the components being analyzed approaches that of the shroud gas, and the shroud gas therefore contributes to the ionization in the sensitive volume, the improvement in sensitivity is retained to some degree.

Also, the distance between the inner face of the collimating plate and the nearest edge of the plate 47 is greater than the range of photoelectrons in the shroud' gas.

The required potential gradient across the plates depends on the rate of gas flow through the chamber and the pressure of the gas. At atmospheric pressure, the mobility of either positive or negative ions is the order of l centimeter per second when the voltage gradient is 1 volt per centimeter. Thus, with a maximum velocity the order of 5 centimeters per second of gas through the chromatograph column and ionization chamber, moderate potential gradients are sufficient to separate the charges in an ionization chamber of reasonable size. If the ionization chamber is operated at a relatively high pressure, say near atmospheric, some re-combination of ions occurs before being collected by the plates. However, a potential gradient of the order of volts per centimeter between the plates is sufiicient to keep there-combination losses small, even at atmospheric pressure. In addition, an appreciable loss of ionization current would result from recombination if the ionized gas were permitted to drift down the axis of the apparatus before it reaches the electric field between the plates. Therefore, preferably the electric field is created in the ionization region as shown in Figs. 2 and 3. With this arrangement, the ions are subjected to the electric field as soon as they are formed, and the plates extend downstream from the UV. lamp so that additional collection of ions takes place even after they have left the region of most intense U.V. radiation.

Preferably, the light from the lamp is monochromatic, and the envelope of the UV. lamp is such to filter out all-unwanted radiation so that only radiation of the proper. energy is emitted to produce the desired ionization. 1T

required, suitable sleeves or filters can be disposed over the lamp to provide radiation of the proper energy. A suitable source of ultraviolet light for analyzing a mixture of benzene and cyclohexane is a krypton discharge lamp whose envelope has a lithium fluoride window so that the U.V. output is mainly 1236 A. and 1165 A., which ionizes the benzene but not the cyclohexane.

An important application of the above described apparatus is in the analysis of exhaust gases from jet engines to determine engine performance. A typical jet fuel has cyclic saturated hydrocarbons in it, and the exhaust has cyclic saturated and unsaturated hydrocarbons. Accurate analysis of the exhaust gives an indication of the efiiciency of the engine, but as will be understood from the above description, conventional chromatographic analysis is not always capable of distinguishing between saturated and unsaturated compounds. However, the improvement provided by this invention increases the utility of chromatographic analysis for such applications.

We claim:

1. Gas analyzing apparatus comprising a chamber, means for flowing a stream of sample gas through the chamber, means for flowing through the chamber a shroud of protective gas which surrounds the gas sample stream, means for irradiating the gas sample in the chamber with electromagnetic radiation in the ultraviolet range of sufiicient energy to ionize at least part of the gas sample, and means for detecting quantitatively ions so produced.

2. Gas analyzing apparatus comprising a chamber in which a sample of gas is disposed, a pair of spaced electrodes disposed in the chamber, a source of ultraviolet light adapted to irradiate gas in the chamber, and shielding means to prevent direct impingement of radiation from the source on the electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,740,894 Deisler et al. Apr. 3, 1956 2,755,388 Weisz July 17, 1956 2,755,391 Keyes July 17, 1956 2,850,641 Martin Sept. 2, 1958 OTHER REFERENCES Chubb et al.: Photon Counters for the Far Uultraviolet, article in The Review of Scientific Instruments, vol. 26, No. 5, May 1955, pages 493 to 498.

Deal et al.: A Radiological Detector for Gas Chromatography, article in Analytical Chemistry, vol. 28, No. 12, 1956. 

1. GAS ANALYZING APPARATUS COMPRISNG A CHAMBER, MEANS FOR FLOWING A STREAM OF SAMPLE GAS THROUGH THE CHAMBER, MEANS FOR FLOWING THROUGH THE CHAMBER A SHROUD OF PROTECTIVE GAS WHICH SURROUNDS THE GAS SAMPLE STREAM, MEANS FOR IRRADIATING THE GAS SAMPLE IN THE CHAMBER WITH ELECTROMAGNETIC RADIATION IN THE ULTRAVIOLET RANGE OF SUFFICIENT ENERGY TO IONIZE AT LEAST PART OF THE GAS SAMPLE, AND MEANS FOR DETECTING QUANTITATIVELY IONS SO PRODUCED. 